Isotropic pitches are used for preparing various isotropic carbon materials, for example, isotropic pitch based carbon fibers, activated carbon fibers (ACF) and conductive carbon materials.
The isotropic pitches have been conventionally prepared by processes in which petroleum heavy oils such as catalytically cracked (FCC) residual oils as starting materials are subjected to heat treatment or blowing method using oxygen or ozone.
In these processes for preparing pitch, however, if a petroleum heavy oil containing light oil components is used as a starting material, these light oil components are eliminated before the thermal polymerization reaction proceeds, and therefore the yield of pitch is very low. Especially when a heavy oil which contains a large amount of light oil components and which is liquid at ordinary temperature is used, the yield of pitch becomes extremely low, and preparation of pitch is practically impossible. On that account, the petroleum heavy oil containing a large amount of light oil components was unable to be used as a starting materiel of pitch for preparing carbon materials and such heavy oil has been conventionally used only as a heavy oil blending component.
It is known that the isotropic pitches are nitrated before carbonizing or graphitizing them in the preparation of carbon materials. Japanese Patent Laid-Open Publication No. 302217/1993 discloses technique relating to a process for preparing low-viscosity pitch for matrix, comprising the steps of spinning ordinary isotropic pitch to form fibrous pitch and nitrating the fibrous pitch in a solid-liquid phase.
In this technique, however, the starting pitch is made fibrous by spinning and the nitration reaction is carried out in the solid-liquid phase in order to conduct an uniform nitration, that is, a procedure of spinning the starting pitch to make it fibrous is necessary, and therefore the process is complicated.
Further, the pitch for matrix obtained by this process has a low softening point and a low viscosity, so that this pitch cannot be applied, as it is, to uses other than the use for matrix, in which a high softening point is required.
For raising the softening point, there is known a method of heat-treating low-viscosity pitch having been subjected to nitration reaction, so as to polymerize the pitch. In the nitration reaction in the solid-liquid phase in the conventional process, however, there is limitation on the degree of nitration, and hence isotropic pitch sufficiently nitrated cannot be obtained. If the isotropic pitch thus insufficiently nitrated is heat-treated, polymerization reaction does not satisfactorily proceed and isotropic pitch having a desired softening point cannot be obtained.
The present inventors have earnestly studied in view of such circumstances of the prior art as mentioned above, and they have found that, by a process comprising the steps of mixing a petroleum heavy oil, particularly a petroleum heavy oil containing a large amount of light oil components, with a nitrating reagent and water to give a homogeneous dispersion, nitrating the heavy oil in the homogeneous dispersion state, then refining the nitrated heavy oil and thermally polymerizing the heavy oil, a petroleum heavy oil can be uniformly nitrated and polymerized with inhibiting elimination of the light oil component. The present inventors have made further improvement and accomplished the present invention.